C3 HDi 16V - Checking VVTurbo operation

[CitTone]

Before starting, lets see if I have this right...

A lot of the time (most, really) I'm getting little boost at low revs, although the car still goes like stink once under way. I've cleaned up the carbon deposits as far as possible on the exhaust side of the turbo, which shows no obvious signs of clogging, and have checked that the variable vane mechanism is not jammed.

The variable vane turbo vane mechanism is actuated by an electrovalve (rear of engine, RHS) which directs vacuum from the vacuum pump to the actuator on the turbo, operating a vertical pullrod. This happens only at low engine revs (<2500rpm?) - correct?

If all the above is correct, how can I check for proper operation? I assume that I need to know :
1. The correct operating pressure
2. When it should be applied
3. What the actuator throw should be

Does anyone please have any of the figures that would help me to analyse the current status so that I have a chance of diagnosing which bit of the system is not behaving itself. Could, I guess, be as simple as a leaky or collapsing vacuum pipe, but I'd like to know before I start pulling things off the engine!

TIA